(1) Field of the Invention
The present invention relates to a method of producing an improved carbon substrate cathode coated with palladium and iridium which is highly efficient toward catholyte reduction and is very stable.
(2) Description of the Prior Art
Aluminum anodes used in aqueous batteries with, for example, silver (II) oxide or air cathodes have been extensively investigated. The use of a hydrogen peroxide catholyte in combination with an aluminum anode has been the subject of several studies. When aluminum is combined with hydrogen peroxide in a caustic electrolyte, the overall cell reaction is:
2Al+3HO2xe2x86x922AlO2+OHxe2x88x92+H2Oxe2x80x83xe2x80x83(1)
Operating cell voltages in the 1.2-1.7V range, depending on current density, have been routinely obtained. The Alxe2x80x94H2O2 electrochemical couple compares favorably with the Alxe2x80x94AgO couple and other high energy density primary battery systems. An energy density of 360 Wh dmxe2x88x923 is projected to be achievable for the Alxe2x80x94H2O2 solution phase catholyte system whereas an energy density of 290 Wh dmxe2x88x923 is projected for a similarly configured Alxe2x80x94AgO system.
An ion diffusion membrane is not used in the semi-fuel cells studied to reduce complexity, weight, and cost while increasing reliability and cell voltage. The approach, however, results in the catholyte being in direct contact with both the anode and the electrocatalytic cathode substrate. A direct, non-electrochemical reaction will thus occur. Unless the rate of this direct reaction is low compared to the electrochemical anodic and cathodic reactions, the direct reaction will significantly reduce the overall efficiency of the cell. Additional reductions in efficiency can occur due to the corrosion reaction of aluminum with hydroxide ions and the decomposition reaction of H2O2.
Corrosion Reaction: 2Al+2H2O+2OHxe2x88x92xe2x86x922AlO2+3H2(g)xe2x80x83xe2x80x83(2)
Decomposition Reaction: 2H2O2xe2x86x922H2O+O2(g)xe2x80x83xe2x80x83(3)
Improved catalysis for the reduction of H2O2 should result in reductions of the direct and the decomposition reactions, thus improving significantly the electrochemical efficiency of the cell.
A magnesium anode and either hydrogen peroxide or sodium hypochlorite catholyte have also been investigated. The overall cell reaction in each case is as follows:
Mg+HO2xe2x88x92+H2Oxe2x86x92Mg2++30Hxe2x88x92xe2x80x83xe2x80x83(4)
Mg+OCl1+H2Oxe2x86x92Mg2++Clxe2x88x92+3OHxe2x88x92xe2x80x83xe2x80x83(5)
Cell voltages in the 1.1 to 1.5V range have been obtained. Like the aluminum-hydrogen peroxide system, these systems also are operated without a membrane and thus have to be concerned with the direct reaction of the catholyte (HO2xe2x88x92 or OClxe2x88x92) with the magnesium anode. The hypochlorite system also exhibits decomposition and precipitation reactions.
Decomposition Reaction: 2OClxe2x88x92xe2x86x92Clxe2x88x92+ClO2xe2x80x83xe2x80x83(6)
Precipitation Reactions: Mg2++2OHxe2x88x92xe2x86x92Mg(OH)2(g)xe2x80x83xe2x80x83(7)
Mg2++CO32xe2x88x92xe2x86x92MgCO3(g)xe2x80x83xe2x80x83(8)
Here again, improved catalysis will result in reduction of the decomposition and precipitation reactions.
Several catalysts have been investigated for use in fuel cells. Many of these catalysts (including palladium and iridium independently) have been incorporated in carbon based pastes. J. P. Collman and K. Kim, J. American Chemical Society, 108:24 (1986) 7847, have reported that iridium porphyrin complexes are very active catalysts. Cox and Jaworski, J. Analytical Chemistry, 61 (1989) 2176, have used a palladium-iridium combination on a glassy carbon microelectrode for the quantitative determination of H2O2. A combination of palladium and iridium has been shown to improve the electrochemical efficiency for the reduction of H2O2 and to improve cell voltage relative to the use of a metallic silver cathode.
Under high rate discharge (800-1000 mA/cm2) efficient electrochemical reduction of hydrogen peroxide (approximately 70% coulombic efficiency) with a concomitant decrease in its heterogeneous decomposition has been achieved through the use of a Pd/Ir catalyzed cathode substrate. Efficiencies 30 to 40% lower are obtained at current densities below 100 mA/cm2. It should be noted that applications of the Pd/Ir catalysis of a nickel cathode substrate have been limited to high rate, single use, short duration (less than 30 minutes) situations. When attempts have been made to reuse the catalyzed nickel substrate, performance changed with the second use and typically decreased significantly with a third use.
The patent literature contains a number of patents related to the manufacture of cathodes for use in chemical cells. These include U.S. Pat. No. 4,506,028 to Fukuda et al., U.S. Pat. No. 4,514,478 to Binder et al., U.S. Pat. No. 5,296,429 to Marsh et al., U.S. Pat. No. 5,395,705 to Dorr et al., and U.S. Pat. No. 5,578,175 to Lin et al.
The Fukuda et al. patent relates to an electrode substrate for a fuel cell. The substrate is prepared by a process comprising mixing 30 to 50 wt % carbon fiber, 20 to 50 wt % of a binder, and 20 to 50 wt % of an organic granule, press-shaping the resultant mixture, curing the shaped product, and calcinating the cured product.
The Binder et al. patent relates to a porous carbon cathode for use in an electrochemical cell. The cathode is made by wetting the carbon black with a 1:1 to 1:3 mixture of isopropyl alcohol:water, adding a binding agent thereto, smearing the resulting stiff paste on a thin expanded metal screen and pressing and rolling to a desired thickness, drying the cathode sheet in a vacuum oven at about 100xc2x0 C. for one hour while a weight is placed above and below the cathode sheet which is sandwiched between two pieces of blotting paper, the total weight applied being sufficient for the cathode to retain structural integrity, removing the weight and blotting papers and inserting the cathode sheet in an elevated drying oven at about 280xc2x0 C. for about one hour, and cooling the cathode sheet between blotting paper, repressing and rolling.
The Marsh et al. patent relates to an electrocatalytic cathode which is comprised of nickel specially coated with one or a combination of platinum, ruthenium, rhodium, osmium, palladium, and iridium. The cathode is produced by pretreating a nickel electrode with a hydrochloric acid bath and then by applying the metal coating by plating methods.
The Lin et al. patent relates to a process for manufacturing an iridium and palladium oxides coated titanium electrode. The process comprises preparing a titanium substrate having a surface, applying iridium and palladium to be formed on the surface of the titanium substrate, and heat treating the iridium and palladium oxides applied titanium substrate to obtain an iridium and palladium oxides-coated titanium electrode.
The Door et al. patent relates to an electrochemical cell having at least one electrode containing carbon fiber paper coated with an uncoagulated mixture of binder and catalytically active metal particles at catalyst/binder ratios of about 2/1 to about 25/1. The metal catalyst particles include at least one of the metals of Group VIII of the Periodic Table of Elements, preferably rhodium, ruthenium, palladium, osmium, iridium, and platinum (platinum black).
Accordingly, it is an object of the present invention to provide a method for producing an improved electrocatalytic cathode for use in an electrochemical cell.
It is a further object of the present invention to provide a method for producing an improved electrocatalytic cathode as above that is highly efficient toward catholyte reduction and very stable.
It is still a further object of the present invention to provide a method for producing an improved electrocatalytic cathode as above which is capable of performing over a wide range of energy densities and is suited for low powering endurance as well as for high power applications.
The foregoing objects are attained by the method of the present invention.
In accordance with the present invention, a method of producing an electrocatalytic cathode for use in an electrochemical cell system comprises the steps of providing a carbon substrate and simultaneously depositing palladium and iridium on the carbon substrate by cyclic voltammetry or by controlled potential coulometry. The simultaneous deposition of the palladium and iridium is preferably carried out using a solution containing 1.0 mM palladium chloride, 2.0 mM sodium hexachloroiridate, 0.2M potassium chloride, and 0.1M hydrochloric acid.
Other details of the method of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompany figures.